When I went for genetic testing, the secular geneticist told me that her rule of thumb was "oh they'll just test each pregnancy and terminate the Tay-Sachs ones." Suffice to say that Rabbi Moshe Feinstein vehemently opposed that logic. His student Rabbi Tendler is therefore opposed to amniocentesis as the couple will then be pressured to terminate -- when according to Rabbi Feinstein it's not permitted to do so. (Note that other rabbis are more lenient in some unfortunate cases.)
I think the majority of Orthodox poskim today will tell you that two carriers really should look for other people to marry. It's the least-bad option. Hence the importance of checking for compatibility well before a relationship gets underway. (Rabbi Feinstein himself recommended finding one's carrier status as soon as one decides that s/he is "seriously of dating age", while the tests could be done earlier he didn't want more anxiety weighing on people. Today Dor Yeshorim removes the stigma by no one knowing their actual status.)
Should a couple realize they're both carriers: Rabbi Feinstein had ruled "if a man has a condition such that, regardless of who he marries, there's a 50% chance that his child will have medical problems but a life expectancy of several decades with treatment ... he should have faith and get married", with others questioning this. That's a far, far cry from saying that a couple who know there's a 25% chance any child they have won't survive preschool age should have "faith."!
- A couple who know they're both carriers shouldn't get married.
- If they're already married and determine they're carriers, it's reasonable to question whether the mitzva to try to conceive conventionally would apply -- as the emotional cost here is so high. I'd assume many would advise against it.
- Adoption, or IVF with preimplantation testing, are both complex options that may be advisable, pending a couple's consultation with their rabbi.