Would merging two coins' network into one to prevent 51% attacks mean the difficulty for mining any of the coins would also be merged (hence increased, as it's all added together)? If two networks merge, will there even be the possibility of mining only one of the two (or more) coins or would it mean you can only mine all of them at once?

It's a very interesting concept indeed. As the posts above said, most small hashrate coins are successfully attacked because the team behind them don't give a damn about protecting the coins. I've had absolutely no respect for XVG especially after their stupid and cringy PornHub & McAfee move.