To make that work in bitcoin you would need to be able to create a private key that both of you know, but a public key that each of you only know half of. In the crypto sense of half, where there are two secrets that can be combined together mathematically to make a third secret.
You would both transfer 5 BTC to the bitcoin address matching the public key. Then neither could retrieve the coins until one surrendered his half to the other.
A proxy comes to mine, but that is cheating. If you used a proxy to generate the private key and split it, they would know the private key and in that case you might as well trust them to hold the money without splitting the key.
I don’t know enough about how elliptic curve based keys are generated to propose an algorithm. But stranger things have happened in crypto than that. In some cases, you can do math with two unknown encrypted numbers and have and have the answer decrypt correctly.
Go figure.