I worked in the matrimonial dept' of a solicitors office some time ago and we had the direction that it was not allowed. The reason given was that there is a conflict of interest between the husband and the wife which makes it impossible; even a potential rather than an actual conflict would render it difficult.
It could be that when advising or drafting an order, the solicitor needs to advise one party that the agreement is not in their best interests. If the solicitor is acting for both of them, which party would he advise? He cannot advise both.
Of course this may have changed and it could have just been best practice etc.
You could check with the law society?
As someone else posted, if you have both decrees, you are divorced!