Hi Yves,
Thanks, your answer brought me a little closer to solution.
I did as you suggested. Now when I login on Q system as user PSIDadm or SAPservicePSID I can access /usr/sapmnt/trans from windows. However SAP on Q won't access shares on P system. For test I created directory in AL11 pointing to \\sapsrvP\sapmnt\trans and it's not accessable. If I try to add parameter DIR_TRANS pointing to \\sapsrvP\sapmnt\trans again it won't work and reports it cannot find this location. In STMS I reconfigured Q system from scratch and put it in the same transport group P system is in.
When I display TMS configuration for Q system I see TRANSDIR set to \\sapsrvP\sapmnt\trans.
When I open TP parameters for Q system I see TRANSDIR set to \\sapsrvQ\sapmnt\trans ?!
Then I created parameter DIR_TRANS with value \\sapsrvP\sapmnt\trans in instance profile but then tp checks in STMS fail. This means SAP is not able to connect to shared directory on P system. What could be wrong?
BR,
Sasha