Can I combine seller financing and mortgage?

[Kathy Moser]

Hey Amazing Real Estate Rock Stars,

I'm doing an off market deal and have been offered seller financing.  

It's a multi house deal and all the houses have cesspools that need to be upgraded.  

The town requires septics to be upgraded within 6 months of transfer of ownership so I had proposed an installment plan offer, doing a house every 18 months and refinancing out to BRRRR the money into the next house. I don't have enough cash to do all the septics, a down payment and still have sufficient reserves.

Just wondering if it might work to get a mortgage for part of it which would take care of down payment and then some, seller financing for the rest and then do all the septics at the same time.  

Does anyone have experience combining seller financing with a mortgage?  Thanks!