I’m afraid there’s no easy way to do it. If you use hot air it tends to damage the plastic.
I got my technique from Kriss (when he was helping me). I use a chisel tip on my soldering iron and just push it underneath each pin. I had to sharpen the tip to make it easier.
I have no idea if this is technically possible. It is not like you can simply flash two VirtualBoy games on one card.
Both ROM’s together should be under 2MBytes. So, couldn’t you put in an option screen that lets you decide which version to run (when you first boot the cart) ?
I’ve built carts for other systems and these are by far the hardest / most time consuming to build. No other repro involves having to buy other carts and meticulously remove the connectors prior to building.