Can I define 2 Stores on StoreFront 3.15? One pointing to XenApp 6.5 and one to XenApp 7.15?

Yes, you can define multiple stores on the same StoreFront server pointing to different farms.
Each store would have a unique website defined to it, and that's how you would send users to the correct site.
If you are using a NetScaler, you would simply code the website name into the session profile.

Some additional points to note:
- You can have multiple websites connected to the same store. This way, you could have the Finance Dept connect to website A (with their own branding, etc.), and Sales connect to Website B (with their own branding).
- While you would need to define different XML servers for the 2 farms, you do not need to define different STAs, as long as you can reach the STAs defined from both farms.

... and yes, you can use a second IIS site (if you wish) to separate the two stores, but you may need to define the second one via PowerShell if it's not on the default IIS site.

... and it is possible to mix XA6.5 and 7.x within the same store.
Usefull while migrating.

Great, wonderful feedback.  So, now to move forward into the mechanics and ensure I do it correctly ;)  Thank you.

You are most welcome.

In working with this more, is there a way to separate the Store completely in StoreFront?  When generating a new store in maintains the base URL as part of its string.

Thanks in advance.

Yes, you can create multiple stores on the same StoreFront server.
The base URL remains the same.

As far as the multiple site options.  For clarity and so I can at some point upgrade StoreFront when XenApp 6.5 is gone, I went the route of seperate IIS Sites per https://docs.citrix.com/en-us/storefront/current-release/plan.html#multiple-internet-information-services-iis-websites and as noted the StoreFront console detects the multi-site configuration and no no longer displays any options.

So, a few more questions.

#1 - How do I populate the IIS Directory for the Second site with the additional items?  Right now just has Citrix\Roaming.  It does not contain any of the AGServices or CitrixStore type content.

#2 - Once you go to multi-site, is there a way to get the GUI back or how to do configure the Store Settings?

#3 - Or is the preferred method a secondary store within the Same IIS site though they are both different farm versions, 6.5 and 7.15?  How do I separate the two for users?

Trying to find the documentation, but your input is appreciated.

Thanks in advance.

Ok thanks. I'll try #3 - so how does that work in the Netscaler realm and routing users to the second store then?  I guess that's why I was thinking a separate site was a good option.

When you create the second store, it will also create a unique website name, e.g. https://storefront.domain.com/Citrix/Store2Web.
You simply put this website name in the session profile on the NetScaler.

Ok, thanks.  Now to reverse this additional site ;)  Good thing it's the test environment and not in use yet ;)  Might have to blow it away and start over. Thanks for all your help :)

Awe, found the syntax and noting it here as "Clear" is not too intuitive, I tried Remove, Delete, Drop, etc.  

To take out the secondary site:
Clear-STFDeployment -SiteID 2

Woo Hoo, thanks, I can now get to the second URL, likely need to deal with Firewall as I'm getting the all too familiar Cannot complete your request, but it does resolve to the secondary site.  Thanks for walking through this with me :)

Sure thing.

Sam, I just wanted to let you know I appreciate all the help you gave me.  I have been able to get this up and running finally so I can access both my XenApp 6.5 and XenApp 7.15 systems from the Same StoreFront system using a secondary store.  I think the main issue here is the many changes in how this all works.  Ultimately it was a combination of things around the STA's (Not understanding how the Delivery Controller's work, how to set the port I wanted to use), along with additional firewall rules and DNS resolution to the Gateway URL from StoreFront.  So just wanted to say Kuddos and thank you for jump starting me in the right direction.  Thanks.

You are most welcome.