I agree with
itareus that double track is redundant and there is no need for it, once you start laying parallel single tracks. Single track gives you more flexibility - you can keep tracks 4 meters apart and widen this space on bridges, road crossing, etc. You can make your routes looking more prototypical by pulling splines on the outer track in tight curves, which you can't do with fixed double-track. Also when joining parallel tracks using turnouts you need to switch from double-track to single-track and your track layout can get a bit messy.
To make double track easy I use following guides from
martinvk (see the picture below showing 5 meters guide). They are easy to use and look great thanks to its distinctive, colorful pin, although I wished there would be also one more guide for 6m track.
Guide 2t 3.5m,<kuid2:70337:23089:2>
Guide 2t 4m,<kuid2:70337:23083:2>
Guide 2t 5m,<kuid2:70337:23086:2>
Guide 3t 3.5m,<kuid2:70337:23090:2>
Guide 3t 4m,<kuid2:70337:23084:2>
Guide 3t 5m,<kuid2:70337:23087:2>
Guide 4t 3.5m,<kuid2:70337:23091:2>
Guide 4t 4m,<kuid2:70337:23085:2>
Guide 4t 5m,<kuid2:70337:23088:2>