I would look to prune out some of the old wood perhaps after the second year when both roses have hit the top of the arch. It would have helped if you had chosen both varieties that had been equally vigourous. The only problem you might experience is that most of the blooms will tend to be at the top of the arch. When roses are grown on a trellis, then every year you bring the pliable stems down to the horizonal and cut out the old wood to encourage flowering.

You may also find they never grow at the same rate if their exposure to sunlight is different.

Hello ... you could try gently bending some of the lower shoots from upright stems into a more horizontal position, winding them into the trellis. Bending rose branches in this way, seems to cause more flower shoots - similar to training fruit trees into different shapes. So, pull down some branches and gently push them down and across sides of your trellis. You will have to do this every year, to keep producing lower flowering branches, and I guarantee some will break. Best of luck.

Hi thanks everyone for your help. Jimmy - both roses are the same variety and planted on the same day, so i think perhaps the different rate of growth may be due to difference in exposure levels as Moon Grower suggests. Although that said, the arch is in a very sunny spot. I will take all your advice and hopefully next year i'll have blooms further down the stems. thanks again.