Depending on how many sheep you have got I would be tempted to keep them on the two different vaccines. However if less than 10 sheep you are probably just as well lambivaccing the sheep. I would do the double-dose, so 6 weeks before lambing for 1st one and the 2 weeks before 2nd one.
You then have to think what to do about pasteurella, but I do my goats on Ovipast separately. My sheep are on Heptavac.