First, a few posts that might interest you, on jacklines, tethers, and the stresses involved. I have a catamaran, so my priorities are different.
Sail Delmarva: More Climbing Gear for Sailors
Sail Delmarva: Sample Calculations for Jackline Stress and Energy Absorption
Sail Delmarva: Climbing Gear for Sailors--Jacklines and Harnesses for the Unemployed
Pictures would help. I'm assuming the forward anchor point is a padeye that you could simply pass the jackline through. Make sure it is MINIMUM 5,000-pound rated and has 2 X 5/16-inch blots through to a backing plate. I don't understand how you are securing the and tensioning the lines (most people use the bow cleat for that reason--this also makes removing the jacklines easy, since nylon webbing should not be left out on the sun; if it is, the safe life of the jacklines is probably about 12 months, based on UV strength loss studies).
* You will have more stretch since a continuous line is twice as long. Not quite twice as much, as there will be some friction through the bow anchor.
* The load on the bow anchor point is about 175% of the basic jackline tension if the line is simply doubled back (that jackline is pulling and so is the dead end). You would need a very strong anchor to account for this stress doubling (about 8,000 pounds).
* The increased stretch will reduce impact forces but will increase the distance you fall. On a small boat and with nylon webbing, I think the reduced stretch is more important; you don't have the long fall distances a catamaran can have and you do have narrow side decks and bow.
* There will be some increase in chafe, since the line will rub under load through the bow anchor.
I think you might be happier with 2 lines, but if ease of removal is a factor, consider a cow hitch at the bow pad eye to reduce movement and partially reduce bow anchor stress (but I'm not saying how much, as the knot will slip). This knot is very small and will not seize up after being loaded.