Hello and welcome to the forum!
Can you clarify what you mean by a "2 for 1 pass" - do you mean a Two Together Railcard? This gives you a third off when you travel together with someone, so it's not quite 2 for the price of 1.
The terms and conditions of the Two Together Railcard do require both passengers to travel together throughout the journey, so what you are proposing isn't in line with these terms and conditions.
Moreover, if you were thinking of buying Cwmbran-Manchester tickets for both of you, your sister would be doing what's known as a 'break of journey' when she joins you at Hereford as this is considered 'starting short' (a kind of break of journey). Some tickets allow break of journey - but not all.
In particular, break of journey is
never permitted on Advance tickets (these are usually the cheapest option if booking further ahead). And although it's
usually permitted for Off-Peak tickets, it
isn't permitted on a Cwmbran-Manchester Off-Peak Return because of the restriction code that applies to this ticket - 8A (
nre.co.uk/8A):
Restrictions
Applicable Days
Monday to Friday
Outward Travel
No time restriction.
Return Travel
No time restriction.
Notes
Break of journey is not permitted except to change trains at an intermediate station or to access station facilities.
You would therefore have to buy Anytime Day Singles in each direction for a break of journey to be permitted. This is much more expensive than either buying Advance tickets in each direction, or an Off-Peak Return.
Therefore I would suggest you either both start your journey at the same station, or alternatively that you buy separate Cwmbran-Hereford tickets just for you, and then a separate set of Hereford-Manchester tickets with the Two Together Railcard. Obviously these are all options which are likely to be somewhat inconvenient or possibly more expensive, but it would be the 'safe' option that meets all the rules.
If you do the second option, I would also suggest you change trains at Hereford. The reason I suggest this is that (as above) the terms and conditions of the Two Together Railcard require both passengers to travel together. Hereford station has ticket barriers and so if your sister's ticket is rejected for any reason (e.g. to allow the staff to manually check for her Railcard), the staff might not let her through without you being present. Although harsh, that could even happen if she showed both of your tickets and the Railcard.