If 2 people are on a valid house title, can one create a 'land contract' with an individual without consent of other person on title?

Are the owners tenants in common?

Where?

Yes, but it wouldn't be legal

I now understand, from reading the other thread, that the state is Michigan.

It's possible. It can get into a very complicated argument about whether the person who signed had actual or apparent authority to sign for the other (agency law). Let's take two different fact patterns:

The person who signed the contract and the other owner are 2nd cousins twice removed who haven't communicated with each other for 30 years, and the non-signing cousin lives on the other side of the country. - probably both would have to sign.

The two owners are married to each other. The non-signing wife was in the same room when her husband signed the contract. - signature probably not necessary.
Somewhere in between? Best to get both signatures to be safe.

If two people own a property, the one person could sell their interest in that property, but it opens up an entire issue of liens on the property.

Example If Scott and I owned a property, I could sell my share of it, but not the entire property without the other owner also signing