Each lender, for the most part, is going to have similar rates. it really comes down to communication and their process!

Would a bank(say like BankofAmer) allow me to assume the low interest rate of another property and blend the interest rate with my current rental property?

The terms of your mortgage and interest rate will remain the same and no refinancing would be required.

Im looking to do a STR in Ocean City Maryland and the vacancy rate, if I'm fully booked in the peak season (May -Sept), would be  58%!.

Insurance has been a big issue in our market this year as companies have jacked rates out of control. 

Lending could be an option but with interest rates high 7, I am looking for other creative options.

There are other loan products out there with a varying degree of rates that could be alternatives, but I'm not an expert in mortgage products so I'd recommend speaking with a lender first. 

What is your current mortgage rate? 

In the vaguely unlikely case that the cap rate and deal is real and there aren't any significant "gotchas," 25% down should be easily doable.

That FHA loan is assumable, so that might be attractive (depending on your rate).You could consider leaving the FHA loan in place and just move into a live in flip.