Could a couple both in reciept of Incapacity Benefit and DLA (mid/high care) be entitiled to the SDP X 2 AND the Carers premium X 2 as part of an Income Support Claim. (assuming both discounted from payment of Carers Allowance on 'overlapping benefit' rules)

Yes, as long as the caring activities dont conflict with stated care needs then there is no legal reason why not.

In a word - Yes, and this applies to both over and under 60's -
Our HB assessors are trained and encouraged to look for claims where DLA mid care/ Attendance allowance is in payment to a couple, but where SDP/Carers premium are obviously not in payment, and all such cases are passed to our welfare rights officers to follow up. This they do by encouraging the claiming of the carers allowance in order to establish underlying entitlement to CA.

In addition, in cases where Income support was already in payment but not including SDP, one of the first questions we ask of the local DWP is did they receive a notification of an award of DLA/AA having been made - if they had, and it was not acted upon, then we immediately ask for backdating.

Now that the upper limit for capital has been abolished, there are a lot of pensioners out there in particular with over £18000/£20000 savings, who will qualify for full HB/CTB when the SDP/CP are added in. The total increase in benefit income for a pensioner couple could be in the region of £210 per week plus HB/CTB which equals happy retirement and a friend for life.

Thanks for the info folks.
Case I had was elderly chap caring for disabled daughter(DLA mid care). Referred to me by his GP re Carers Allowance claim. Given his own health problems I suggested a claim of AA for himself. High rate awarded within 3 weeks. Contacted DWP re daughters Income support and she had SDP added(both in household now having qualifying benefit). Clients Pension Credit award then included the Severe Disability element.
Then thought of the original referral and the issue of Carers Allowance. Rules would suggest that SDP would be lost ONLY if CA was actually in payment.In this case the father could establish an 'underlying entitlement'(overlapping with Ret. Pension) and then have the carers addition also included in his pension credit entitlement.
Sounded like a 'to good to be true' situation but by what you say all is appropriate. One to look out for fellow advisers!