I think you need to think about other aspects of this question than just finishing weights.
As you can't separate your ewe and tup lambs, then by leaving them entire you are risking the ewe lambs being impregnated by the tup lambs. This can happen from about 5 months. There are three possible results of this.
Firstly, if you are keeping the ewe lambs then you will get a few which will lamb late winter, need housing, extra feeding and so on, which will be difficult as you won't know they're in lamb. This would put your breeding plan for the following year out of kilter.
Secondly, if you are selling them on as stores, then ethically it is unacceptable to pass on ewe lambs which you know are at risk of being in lamb to keepers who will not know that, and will then have the problems you would have had if you had kept them, except you had the advantage of knowing the risks.
Thirdly, if the ewe lambs are going for slaughter at the same age as the tup lambs, then they just could possibly be in lamb. This would mean that the slaughtermen would have the horror of finding live lambs when they gut the animal they have just killed, and the problem of how to deal with that. I know they hate finding that, and it's not much fun for the foetuses either. Also, if they are in lamb, then your ewe lambs would not make your required weight as they will be trying to grow lambs of their own as well as grow themselves.
The obvious way around this is to castrate all your males, until you have facilities to separate the two sexes.
Oh yes, you asked what others do...we separate out all our males at 4 months and they go into a field with our stock tups on the other side of the road so there is no chance whatsoever of them getting together with any females (including our neighbour's as we have planted a double fenced hedge which they can't get over or through).
We only band those tup lambs we know we don't want to assess as breeding stock later (ie they have a fault visible at birth), but then we don't breed our sheep primarily as meat, but as breeding stock. Any tups which don't make the grade go in the meat queue for slaughter at 16 months. We rarely slaughter ewe lambs but when we do they stay in with the ewe flock until they too are 16 months. At tupping time the ewe lambs are kept well away from the breeding groups.