Welcome to the Three Community.
That's a great question. I'm interpreting this as the PAYG SIM is coming with you to the EU, and you'll be leaving the pay monthly SIM in the UK?
If that's the case, then when someone calls your pay monthly, it will be making a diverted call to a UK number so as long as you have an inclusive, or unlimited allowance the diverted call is covered.
There's just one thing you need to be aware of. Whilst PAYG accounts do get to roam without the daily charge, it is against our fair use policies to take a PAYG for the express purpose of having a free incoming line abroad (See your price guide for more info). Make sure that you top up before leaving, and make at least some level of usage on the PAYG, so that this doesn't flag up as abuse of the service.
Hope this helps.