You dip spellthief 1, then go into a wizard / other caster UM. The spellthief has no casting yet, so does not effect UM. This allows you to take the master spellthief feat. This is what you really want. It stacks Caster level for ALL arcane casting classes. The wording can be read to mean one of two things.

A - your levels in all arcane casting classes stack for arcane casting caster level
-You sum your levels in all arcane casting classes and compare that to the CL you would have in each class at that level of purely that class.

B - your levels in all arcane casting classes stack for arcane casting caster level
- You sum your caster levels to determine your total caster level.

I find myself leaning twards the first reading, but I have known people who lean twards the second. Ask your DM for clarification.

This handily gets around the caster level split that grants a casting level to your LOWEST caster level each level, as all your caster levels are the same regardless of whatever class you have more levels in, so you can place that level wherever you wish (you want to max out your wizard casting).