A series of manganites Tb1-xHoxMnO3 (0<=x<=0.6) with orthorhombic structure are synthesized and detailed investigations on their multiferroicity are performed. Successive magnetic transitions upon temperature variation are evidenced for all samples,
and both the Mn3+ spiral spin ordering and rare-earth spin ordering are suppressed with increasing x. Significant enhancement of both the polarization and magnetoelectric response within 0.2<x<0.4 is observed, which may be ascribed to the competition possibly existing between spiral and E-type spin orders. Theoretical calculation is given based on two eg-orbital double-exchange model, and the result supports the scenario of the multiferroic phase separation.
