It is highly unlikely that the plan would honor a DRO as you described. But a little change might work. The DRO acknowledges that the form of benefit is a 75% J&S. Let's presume for argument's sake that the Single Life Annuity would have been $1,000/month but the plan is actually paying $920/month as a 75% J&S. While both are alive the plan pays $460 to each. If participant dies first, the monthly amount payable to the alternate payee changes to 75% * $920, which is $690/month. If the alternate payee dies first, the monthly amount payable to the participant changes from $460 to $920. Easy, peasy. Note that the plan pays the same amounts no matter the marital status of the recipients.
If you want to change recipients for a portion of the monies then you'll have to horsetrade one thing or another.