As Stan says, there is no problem,...but,...why on earth???
As FF says, just get your CPL out of the way as cost effectively as possible. If you are going to make mistakes make them at £100's of pounds rather than multiples thereof! (excuse the pun!).
I suppose it depends on what you intend to do and how much spare cash you have to burn, but the CPL is a pretty intensive course without having to worry about an extra engine etc etc!
Get it out of the way then do your MIR as a separate entity would be my advice...but I could be wrong, as we will no doubt hear!
Good luck!