Is there a way to give titles to independent lords?

The only workaround I could think of is to merge realms through inheritance (accelerated, if possible).

If you want to grant a second title to a person you just made independent, grant that title to one of their parents or childless siblings instead. Then wait until that second person dies so the intended recipient inherits it. This, of course, only works when they are the primary heir according to the applicable succession laws.

When you have a woman with a title (which many religions make impossible or unlikely) you can also arrange a bethrothal with a boy from your court who stands to inherit another title (matrilinear if you have to). They won't merge their realms while they live, but when they have a son, that son will stand to inherit both titles... unless they have gravelkind succession and get a second son.

The last straw would be to hold on to a title they have a strong claim on and hope they declare war on you for it, in which you promptly surrender. Ambitious characters and characters who dislike you (which goes against your goal to form an alliance with them, of course) are more likely to do that in general, and irrational characters are more likely to try if they have less soldiers than you.