To my understanding players can have partial contracts with companies. Such as a contract for putter use, irons, wedges, drivers etc. Top top names on tour have the most market value (obviously due to exposure and generally get larger contracts covering the entire bag). But all in all, most of these players would say they just want to play what works best for them regardless of company.

It is possible to have mixed bags. Note the number of players that have clubs from companies that don't offer a ball that have Titleist balls.  Like the current Master's winner.

The 20 year old with a Titleist contract but apparently not a FJ contract, at least for clothing, has a white fairway club.  The last 2 #1 world golfers kept their Cameron putters after all the other clubs blew into their bags.  One manufacturer claims they have a hybrid in more bags than anyone and not many pros on the regular tour have a full bag.  I expect if your contract is for 10-20 million a year and the tour van custom makes you a club to your specs, you'd be a little more loyal.  PM and I are both LH, but Titleist treats me better than his supplier for available LH clubs. 

I think mixed bags are pretty common.  I noticed DJ putting with a Scotty yesterday, and he's on every TM driver commercial they make.  

I should have phrased that differently. I was just surprised Todd plays all TM clubs, yet has a Titleist/FJ combo. 

I have read where some contracts for clubs are for a 10 club minimum, depending on the $$ value and length of contract. I would think when it comes to Putters any pro would prefer a Scotty whenever possible.