You might consider paying off your mortgage and then using a line of credit on the paid off house to borrow the money (at normally 1% over prime) as down payment on the new property. You can also get loans using BTC as collateral if you are not already aware.
House as collateral is possible also, but depends on the deal I could get and what the tax implications would be. I haven't actually looked deeply into using BTC as collateral - might be something there.