From my experience I would start out with 4 files on both instances and would have a dedicated RAID group for TEMPDB. With multiple instances and both having high performance OLTP databases you are bound to have contention with TEMPDB if it is configured by default out of the box. If this system has been up and running for awhile you are in much better luck because you have some data to go on with the size of TEMPDB that you need and possible work load that has been put on it. Just make sure you are following as many of the best practices as possible. Things like moving the ldf and mdf to dedicated drives, sizing it appropriately, splitting the data file into multiple equal size files, turning off auto close, etc. There are lots of resources about TEMPDB out there.
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