Re: two schools?
I train at two dojo. Both sensei are cool about it. Both sensei know I do it. They happen to be in the same organization.
I happen to have two teachers that are very open-minded and want the students to train with various instructors. I agree with their feelings that tunnel vision can occur while training with the same people all the time.
There are many other factors you haven't mentioned. Being a school teacher, I find I answer questions with questions, so bear with me and maybe think about the following:
What do you REALLY want out of your aikido training?
How badly do you want to train more than once a week?
Does the other dojo have many more classes that fit your schedule better?
Do you have a long history with your current sensei?
Do you feel there is a lot more you can learn from him?
Do you feel you have an obligation to him, his dojo, and the organization?
Do you enjoy his teaching style?
Do you enjoy the curriculum (both stated and subtle things taught)?
Do you think training, or even visiting, other dojo will hurt relations or even burn a bridge?
Do you think your sensei thinks of your best interests?
Have you experienced other styles/flavors/organizations/etc. and, if so, do you feel you found the one that fits you the best?