there is already an alternative tax-rate you can set for any item

>Is this possible?
in short: at the moment, no. there is no “combined menu” possibility/plugin – or whatever you might want to call it – in existence. (Not to my knowledge anyway)
One could probably – in theory – create some discount of some sort for this depending on what’s been put into the cart but I hazard a guess the idea is to be able to offer a “2 schnitzel with 2 beers” – i.e a combined menu – sort of thing with different taxrates depending on what’s part of that menu
this is currently not implemented (and no plans exist to do this in the sort or even medium term future from my side)

I’ll make a note of that scenario as I understand the issue, but cannot promise that it will ever make it anywhere near the top of things to do I’m afraid