Yes, only had to cut one hole for the second exist. Although I had to get full back box cut off due to the standard twin tips being too wide to the fit the Rieger’s single exist as Rieger has 100mm exist both sides roughly and the tips together are around 140mm roughly.
Im not sure if Rieger does a standard exist diffuser, I know you can get a central twin exist if you want to make it like the mk2 Cupras. But I do know Maxton makes a diffuser for standard exist, but Maxton’s twin single exist diffuser doesn’t work for Pre-Facelift due to it replacing the standard diffuser on Cupras and Mk3.5s. As mk3s only have plastic along the bottom part of the bumper which was replaced by a diffuser 2017+ (Cupras always had them).
So essentially yes there ‘should’ be no cutting involved with a twin single exist. Possibly the diffuser itself might need trimmed to
help it fit better, but no cutting of the bumper.