Yes, you can find out if you can use your GI Bill benefits for college as a Veteran, service member, or even a qualified dependent.  You can find payment rates for undergraduate and graduate degree courses at institutions of higher learning.
Is it even possible to take classes at more than one school at a time?
You it is possible to take classes at more than one school under the GI Bill as long as classes at both schools will count toward your degree and the school giving you your degree considers the classes to be required to get your degree.
What about left over money from GI Bill?