A Danish study in Human Reproduction in 2006 found that fathers who naturally had monozygotic (identical) twins or dizygotic (fraternal) twins had "better semen quality" than those who didn't have twins. Further to that, a 2004 study in the Journal of the National Cancer Institute reported fewer twins occurring among subfertile men. So while there is no guarantee that a man who has twins will have better sperm than one who doesn't - on average the man with twins will have better quality sperm.
Camilla Asklund, TK Jensen , N Jorgensen, A Tabor, L Sperling and NE Skakkebaek (2007). "Twin pregnancy possibly associated with high semen quality". Human Reproduction, 22:751-755. European Society of Human Reproduction and Embryology.
Richiardi L, Akre O, Montgomery SM, Lambe M, Kvist U, Ekbom A (2004). "Fecundity and twinning rates as measures of fertility before diagnoisis of germ-cell testicular cancer". Journal of the National Cancer Institute, 96:145-147.