How is rental income taxed if I own with someone else?

Split according to how you own it. Joint tenants are taxed 50:50, tenants in common are taxed in the shares you actually hold.

Married couples and civil partners are taxed 50:50 by default whatever the underlying shares, unless you hold as tenants in common and submit a Form 17 declaration with evidence of the real split.

That declaration only works going forward from the date it is received, so it cannot fix a year that has already happened.

Where one of you is a basic rate taxpayer and the other is not, the split matters a lot. It is an accountant’s question and worth asking before the tax year, not after.