If I'm using a single rope to abseil then I can use a prussick loop as a safety backup. My question is how can I use a prussick loop when I'm abseilling using a doubled rope?
Wrap the loop round both ropes.
Id treat the double rope as a single and tie a french prussic round them both.
As above.
Alternatively, dont let go of the rope. In many years of climbing I never met anyone who used a prussik or any other backup. Carried them with them- yes. Actively used them- never.
have you considered looking at mechanical devices- Petzl 'Stop' (or a double rope equivalent) etc?
petzl shunt is double rope equivalent and works well if need to lock off to clip anchors on very steep stuff, retreating in very bad weather, trying to locate next rap' point etc not that heavy to carry if expecting an ab in/out or problematic retreat
set up is not easy but once you get it works a million times better than prussik knots - cost less than 1/2 a tank of fuel
and always always always always tie a knot in the end of the ropes
yes mostly on big walls / sea cliffs / unknown territory / stormsActively used them
prussik on double not much friction but equal load don't aleways expect to lock when need to
prussik on each single very hard to move and can load ropes such that pull one down thru anchor - so slow that you'll stop using
edit link [url] http://www.planetfear.com/item/Brand_petzl-shunt-ascender_0_0_18925_1.html [/url]