Have 2 tentative Job Offers (TJO) and accepted both. need advice.

TL;DR: I am onboarding for two TJOs (Job A, Job B). If Job A completes before Job B and I accept, can I later switch to the Job B a few weeks later? TJO for Job B was received two weeks after Job A. Thanks in advance!

Hello, I have two tentative job offers, accepted both, and undergoing the hiring process. Job A is GG-13, Job B is GS-13. Job A a potential bonus and probably will start at least at step 7. Job B has no bonus and most likely be GS-13/5. Difference is a 40 minute commute to Job A and 5 minute commute to Job B. I'm torn because on the one hand money, while both jobs are rather technical, Job A is probably more demanding as it's for one of "those agencies" whereas Job B is just DoD. If onboarding for Job A completes before Job B, and I work at Job A but realize Job B is better and work there, are there any repercussions for working Job A for a month or so? Ideally I would like to see both before making a decision. Job A allowed me for salary negotiation. Job B didn't really stress it at all other than ask for my pay stubs and what step I wanted. I have done Security for Job A and just got notified for urinalysis. Job B did not ask for any Security update other than what I had currently and already completed my urinalysis. Anyone in this situation before? How did it go for you? Thanks for reading my really long question. I am very fortunate to be in this position.