I did twin/IR first. You used to have to have done the twin training and passed the skills test before the IR test.
I didn't have much problems to behonest but...
The school I was at part of the deal was you got free solo time on the FNPT II and I did 50-100 hours on that solo.
The next thing you have to deal with is getting your looking out the window head back on for the CPL and also single engine work.
After a couple of hours in the single and finding out the differences in test I said sod it and did the CPL in the twin as well. Didn't cost that much more and I think it would have cost me more in additional training to get all the single work up to scratch.