i would give enough space for them to spread... it all depends on the size you want them to attain (maybe 4" or 5") the star polyps will spread quickly... just my opinion
You could try putting a small rock between them. When the GSP climbs over take it out for trading frags. Replace with another small rock.
Hard to say which one would win that battle. GSP spreads quicker but the ric packs a bigger punch.