I think this depends on where things are.
Is your set up like this?
If the above is how you have things setup, then from a DR standpoint, I would set it up to be as follows.
SERVER1\INST1 -> SERVER2\INST1
SERVER1\INST2 -> SERVER2\INST2
This assumes that SERVER1 and SERVER2 are on separate hardware, and hopefully separate data centers. If this is the setup, then this would give you a setup that is easy to follow and document.
Now, I agree with some of the comments that have been posted. If these are virtual, there is not a reason to stack instances. Each instance should be its own VM. This simplifies your AG setup, your server provisioning, your troubleshooting, and your life.
One big thing you should check with your licensing on is IF you can not pay for your secondary replica. In most Microsoft agreements, if the secondary server is for DR purposes only, then you can have it setup and not pay for the licensing of that node. This lends to the set up I outlined above, keep everything on SERVER1.