Glad you clarified, Franz.
It is very nice that you accompany your friends. . .so let them have the privilege of accompanying YOU to Mass, before or after you’ve joined them for THEIR services. That way, you have fulfilled your obligation (though personally I find it a JOYFUL service, not a burden) and you have shown Christian brotherhood in both giving your attendence to them and accepting THEIR attendence for you.
Or, to put it another way, you have the great opportunity of weekly Mass attendence, participating in the ongoing sacrifice and redemption and partaking of the Body and Blood, Soul and Divinity, of Jesus Christ. . .and, as an EXTRA, as an OPTION, you can choose to also attend a Protestant service.
Like going out to dinner and having a nicely balanced meal and a dessert. You get everything you need at the meal, and you don’t NEED the dessert. If you had the dessert alone, you might get SOME nutritional value but you sure wouldn’t get the proper nutrition and over time you might suffer physical complications. Yet, the dessert in itself is not necessarily a BAD thing (it might be a nice fresh fruit). . .but it is not equal to or comparable to the lovely full and balanced meal that you DO need.
Mass is the meal; your participation at your friend’s services is dessert. Strictly optional, not necessary but probably enjoyed now and then, not as nutritional yet related in a lesser but still “good” way.