Perfect Fifth Double Stops on Strings
There are several disagreeing statements in orchestration books about perfect fifth double stops on string instruments that don’t involve open strings. Some claim they are impossible and should be avoided while others say they are possible. Even talking to different musicians will give different opinions on that matter.
In my experience it is possible with limitations but I rather prefer to avoid such double stops. The problem lies in the fact that the player will need to press two neighboring strings down to the fingerboard at exactly the same spot which is usually an area that is too narrow for two fingers and for some players on some instruments in some ranges too wide for one.
While in the lower positions (particularly on violins/violas) where the strings are closer together on the fingerboard, one finger pressing down two strings might work for most players, the higher you get and the further apart the strings become the trickier this gets and musicians with slim fingers will struggle or find it impossible to create a decently sounding double stop. Also be aware that the one finger presses down two strings method is quite uncomfortable and gives less controll over intonation and phrasing such as vibrato.
So if you’re writing for a solo player and want to involve such fifths, talk with him/her to figure out if they are possible. In a section they will probably automatically divisi such things anyway which is also what you should not comment on. Even if these fifths can be executed successfully, they will have intonation issues and generally not sound that great. So the bottom line here is: usually they are possible but not really desirable.