IF both the tickets are against a single PNR/different pnr and is an i-ticket, you can request the TT to let you both travel in a single seat.
If both the tickets are in different PNR and is an e-ticket,
You cannot travel together in a single seat!
IF...
more... both the tickets are against a single PNR and is an i-ticket, you can request the TT to let you both travel in a single seat. If he does not allow, get a certificate endorsed by the TT and send it while filing TDR!