You can specify 2 breaks, up to 48 hours each.
To get a good price, you often need to fiddle with the connection times.
You can also do some doubling-back, so you could have a single 4-day break if that was any use.
(e.g. first break at Köln hbf, then take the second break at Köln Messe/Deutz, which will double-back through hbf - you can just ignore the trains to Messe/Deutz).
I've done 2 breaks with a 'Special' ticket, although not a 'London' one.