How we can assign a fixed amount of memory for a multiple instance in SQL 2014

Hi,

We have a SQL enterprise server 2014 with two installed instance.

First instance for SharePoint 2013 DBs and second instance  Dynamics AX 2012 R3 DBs.

The server has 32 GB of RAM, currently the process usage for first instance is 26 GB and second instance is 700 MB.

Which mean that SharePoint DBs in the first instance is consuming most of the RAM.

My question is there any way to assign a fixed RAM usage for a specific instance without affecting the other instance?

for example 16 GB only first instance and 16 GB to second instance.

regards,