Do I need 2 mobile phones?

One for each ear? 

We used our UK contracts over here for 9 months before they expired, 3 & Vodafone, no problem with either network.

Different networks have different rules on how much data you can use in another country, some just say fair use!

As for talk time, again I would make a quick phone call to your provider to see what your tariff includes.

I travel to Spain about 7 times a year and never had an issue (EE) but again depends what you use I suppose 

EE is a quick phone call to 150

I was talking to a man who had two chips on the same phone. One for Spain and one for home. 

Thanks Sean. Need to look into it. 👍😊

It's called a dual SIM mobile phone I believe.

I had one a few years ago and found it a pain to switch between sims. Newer ones may be better? 

Back to original question though, unless you have a really draconian contract you should be able to use UK sim  ok. I used my UK 3 sim for 6 months and never went over monthly allowances.

Yes you can use your UK SIM card for calls within your allowance.

If you start to ring Spanish landline numbers or Spanish mobiles with your UK mobile then the call costs will be massive.

That is correct, on my rolling contract , i am with talk mobile (vodaphone) i can call uk landlines and mobiles all part of my contract to uk  with no extra charge .i cannot call a spanish no as that is counted as a international no, or any other european no. YOU will be charged,,, i have a dual sim phone (two sim cards in one phone) its  very easy  having said that its a top  up pay as you go. Ive found a dual sim card phone brilliant. I have a international sim card also which i can phone in spain .. no problems.

Like I said look into EE my contract includes roaming in Europe including calling uk numbers and Spanish numbers, date included too