What advice can anyone give me? (work related)?

So I have a job at a veterinary place, it isn't full time but pretty close, no benefits only pet benefits and it's been a struggle financially to take the job, I finally got an extra day added on and that sure helps but it isn't 5 days a week yet just four. I LOVE the job. So my work schedule has been pretty much been a set schedule so I thought I'd apply for a 2nd job to help get more income in, but don't want that first job to know in case they think I don't like my job or in case they think they would be working me too much-or anything related to that. I finally got a 2nd job and I just did the drug test and the 2nd job keeps asking my availability and keep saying "that should work." It is limited availability due to the 1st job. Anyway the 1st job asks me to switch shifts with other people once every month or so and I don't mind. Well they asked me to work saturday evening instead of saturday morning but i just got done telling the 12nd job i work saturday morning and can work for you evenings and if I switch I may "burn the bridge" for the 2nd job. Honestly I am bad at saying No and turning someone down, I hate it, I like to do favors for people and not have worries. How can I handle this professionally? what would "you" do? Is it worth it to do 2 jobs or should I stick to just the one job? Personal opinions needed. Help!