Are there any cons to use a T-fitting into two ball valves instead of one combinated (Y-valve or similar) valve?
I assume you are asking to be able to independently turn the water off for the sink or the washer. The only issue you could have with a T is running water past the right angle connecting pipe. If you do this run water line in into the singular and let it split out to the other two ends that are in line with each other. (See the picture below)

The reason for this set up is what is the siphoning effect , as fluid passes the singular it will have a tendency to create a suction - siphoning water from that line , also no resistance one way could allow more water to flow in one direction (unrestricted) and not flow in the other - so if you follow this rule as per the picture - you should not have any issue.