I will be applying for a schengen visa for 2 trips to Spain one for 8 days (23rd jun - 1 Jul) and another for 7 days (15 Jul - 22nd Jul), so I will apply for a multi entry visa. On the visa form it asks duration of stay would you put 15 days or would you put 30 days which would cover both trips but of course you would not be spending all of the 30 days in Spain. (If that makes any sense)

 

Thank you for any help

Tourist visa or visa for the family member of a EU citizen? Both are issued as schengen visa.

In our experience you need to state the actual days the applicant intends spending in the Schengen Area, giving the dates of the two trips. Two Consulates issued my wife with a visa covering the number of days we intended spending in Schengen, both multi entry, whilst a third issued a visa valid for three months shy of the expiry date of her passport, thus allowing multi entries over an 18 moth period.
So in your case I would ask for a multi entry visa outlining the dates of the two trips, if she states that she intends visiting at some time in the future and asking for a longer term visa she could strike lucky and get a visa for a longer period, she must have three months remaining on her passport whilst in the Schengen Area.   

The number of days is per entry though one can never exceed more than 90 days in each 180 day (rolling) window.

 

But there should not be much harm if you fill the form out differently such as 'x + y' or both stays combined. If you explain your travel plans clearly and make sure dont fill in a too low number you should get a visa that the very least is not valid for less than the embassy intended to issue. 

 

The Spanish are not known for their generousity or flexibility so if this is your* first visa they may simply give you one entry... 

 

*you write as if you are the applicant, though I assume its actually your partner that needs the visa and thus is the actual applicant. If you are an EU national and you are married the visa should be issued ASAP, for free, minimal hassle and such as I think you know from various previous topics and the Schengen sticky.

Oh and I think you are aware but where I say EU national it means EU/EEA nationals except (in this case) Spaniards.