Definitely punch another hole. ABYC rules aside, you can run into a myraid of problems doing this:
1) Back flow of cooling/exhaust
water from one engine to the other
2) Depending on where you "tee" in, backflow of seawater in followin seas
3) Backpresure and/or throughput capacity of the main exhaust while both engines are running
You'll have to punch another hole, or go for a dry exhaust system (which most cruisers aren't fans of)