I have two homes with ST in both. I linked Alexa and put each device into groups associated with the echo in each home. However, commands to Alexa in one home can still control the other home’s devices. Commands like “Turn off all the lights” will turn off the devices within its Alexa group, but also the ST devices inside other home group.
Is there any way to fix this without a fake Amazon account on the second home to prevent Alexa from doing this?
Unfortunately Alexa doesn’t have a concept of multiple locations like Google Home, Apple Home and SmartThings do.
I have found this to be less of an Alexa issue than a SmartThings issue. Can Google Home now really control devices in only one SmartThings location? Because that wasn’t true last year. If you said “Hey, Google, turn on all the lights” it did ALL the lights in both locations. 
ah, yeah. I read the OP too quick. I think the problem is two fold:
As a Google user, I believe the reason the perceived weaknesses with the SmartThings/Google integration are even regarded as an issue is because the Google end is so badly broken.
Google Assistant works at a Google account level. Running on a mobile device it sticks to that one account and it is capable of figuring out that it should only be interacting with a single ‘home’ (basically a location) at a time and will actually try and clarify which one you mean. It is imperfect but it shows a degree of understanding of the problem.
Sadly the Nest/Google speaker devices are pretty reckless with security and privacy. Despite being explicitly assigned to a ‘home’ they have little or no home awareness and will access any and all the devices they can. Worse still they will happily use the permissions and settings of other accounts that are linked to the device in fulfilling requests. So you get the nonsense where user A can access user B’s personal Routines without knowing they exist.
The other frustration is that devices and scenes linked from SmartThings are only accessible to the user that linked them unless placed in rooms in homes. Unfortunately you can no longer put scenes in rooms in homes. Google confounds you at every turn.
Short version given your constraints (without creating dummy accounts to…)
Probably not
Also given the limitations everyone has mentioned. It may still not work the way you want. The only way I’ve been able to do anything like what you describe is having multiple unique st accounts with multiple Amazon accounts then linking everything up as appropriate then disabling a bunch of devices in the other user’s Alexa tenant. Even then it’s kludge-y.
So without your constraints. May still not work right.
This is one of the tent pole issues many are still irritated with SmartThings on. Allowing users to filter what devices are presented to a given Amazon account has been a feature promised since the day they moved to the current iteration of the Amazon ST skill. Back in October… 2018.
Thank you all for explaining the background issue. It’s good to know I’m not the only one with this limitation.
The weird thing is this architecture has changed multiple times since the echo integration was first introduced, but never to the full satisfaction of the customer base.
These all assume that your smartthings devices are on different locations but the same account. Obviously, if you’re willing to set up two smartthings accounts, you just keep everything separate
Initially, it would only work with devices at one location. You couldn’t get it to recognize the other location at all, even with a separate Alexa account.
Then, without notice or documentation, it changed so that it would only recognize one location at a time, but it would randomly choose for itself which location that was, and it would randomly switch between them. You can imagine how much fun that was! 
Then there was a very brief period of time ( again, without notice or documentation) when you could select the smartthings location you wanted to use. You couldn’t separate your Alexa accounts and make this work, but at least you could limit which smartthings devices were controlled. But this option only worked for a few months.
And then…in 2018, The big change which affected anything using OAuth, including Alexa, Google home, and Ifttt. Again, the change occurred without notice or documentation. But now everything on your smartthings account was exposed to the other service, with no ability to denote by location. Many many complaints and some dangerous situations (such as an extended family with three homes on one smartthings account discovering that the mother-in-law‘s oxygen machine had been turned off by “lights off” command in one of the other two houses.)
At first, smartthings engineering staff posting in this forum agreed it was a problem and said it would be fixed “soon.” Which ultimately became never, and the smartthings staff person collecting the data has since left the company.
So it seems unlikely that they are going to change this unless they go through another major architecture change. 
On the Google side, I have come to the conclusion that under no circumstances should you ever use the same Google account on two or more separate ‘homes’ if you want them to remain separate. If you do you may find you are creating a bridge between those ‘homes’ for yourself and other users that is difficult to break.