Unfortunately, it's not a simple question. A B2 visa will allow you to enter the US for a period of up to six months (possibly as much as a year), but that is subject to determination by the immigration agent on your arrival.

The B visas come with a "presumption" that you actually want to stay in the US forever... which you must overcome by "proving" to the immigration official that you have compelling reasons to leave when you say you will: your home in Spain, family and friends in Spain and/or the UK (or wherever you are from), business interests that would require your return, etc. etc.

It is possible to do the "six month splits" as you are proposing, but each time you enter the US you have to convince the immigration officials again that you are indeed going to leave on time.
Cheers,
Bev