Good morning Grant,
One of first things I tell someone starting into Doubles, be it Overhead or Spey types is to forget all you learnt about fly casting and rods etc as all that belongs to the single handed rod World, you are now entering the World of Doubles where there are a complete set of new concepts and rules.
Its almost as if you were compairing single handed fly rods to spin rods and gear, its just very different.
And mate, thats just what you have done, confused yourself by thinking single handed ideas when your holding a beautiful Double rod.
To try and clear away the confusion,
lines
they are heavier for all Double rod applications than single handed rods,
eg, 8w single handed line should be approx, 210grains
8w line for Double Overhead casting approx 460grains
8w line for Double Spey casting approx 525grains
Therefore if you wish to try a single handed rod line on your Double you would be looking at about a 11w AT LEAST.
The single handed D/T line, compaired to a Spey D/T,
Normally they is a longer taper built into the line, and of course its heavier.
A little trick, you may chose to try.
In your case of a 8w rod.
Find/obtain a 10w D/T single handed line.
Attach a loop to the end and then find another old fly line, and chop off about 3 to 4 feet from the end, the reel end, this line will be level, and attach it to your D/T line via a loop to loop connection, and you have now built yourself a Spey type D/T line on the cheap.
The huge advantage with this system is you can now test different ideas on casting and loading for your rod, you can also remove the level line, weight it and then obtain a section of T8 and T14 line, cut these to the same weight as the level fly line, and now you have two different density sink tips for different current speeds when your fishing, all very cheaply.
Reels bigger line will now require a larger reel.
Trust I haven't confused you any more?
Kind regards,
UB