How much memory does SSIS need? (The real answer is: It depends.)
However, Jonathan Kehayias has a formula that is pretty good, or so I think, since it works great for me.
The simple formula for reserving space for needs other than SQL Server in Windows is:
Start with 1 GB, plus 1 GB for every 4 GB in the machine between 4 and 16 GB, plus 1 GB for every 8 GB in the machine above 16 GB.
After that you should do some monitoring to determine if you need more or less memory for your SSIS packages. That is the it depends portion of your planning for memory use. If you see problem with your SSIS processes then you need to determine if more memory is needed.
Because you have two instances on the same server this will likely require further tuning.