Cenvet credit of service tax paid on rent

No he cannot claim cenvat credit in the ratio of 2:1, because invoice is raised in the name of "A " 

alternatively A and B can asked for two seperate invoices for the same property which clearly mentions 2:1 ratio then they both can avai the respective credit.

Proper documentation can avoid disputes under cenvat  credit . If B takes credit from bill raised on A, department may challenge the genuiness. Further, under CCR, the burden of proof lies upon the assessee. Better to get separate bills as suggested by Shri. Rajesh Choudhary.