Assuming you've configured the multiple files appropriately (see all the blog posts over at SQLSkills.com on how to do this), no, there is no negative impact combining these two. If you set up tempdb appropriately, you're helping SQL Server manage tempdb. Anything, everything, that goes into tempdb from that point, gets that better management.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning