It took quite a bit of work, but Tyrian Arcade is playable from the 2P side alone now. Some things are still acting up a little, but the game doesn't blow up horribly anymore (no collision, lots of effects hardcoded to only work on p1)
https://is.a.qute.dog/media/GgujGn4Qc17jY2wZ4bo
https://is.a.qute.dog/media/iPfJvUNvIcUjt5VIeVA
https://is.a.qute.dog/media/10Px0bpmKLv_zsVbdIs
https://is.a.qute.dog/media/cGlEMyQQiC8rkoQBpsA
(I think the screen tearing is an artifact of xfce4-screenshooter, by the way.)