It is not about Tom or Jill, it is about the company/dept involvement and planning. Seems like the recruitment really went askew in this example. I mean, if you cannot afford their failing time, bad git merges and the poor quality time once the guys are onboard, then it means you should have invested more time in filtering them out beforehand. If you can - then be consequent - plan the onboarding process better. Make a small workshop for them, have two persons (one senior and one junior) spend time to patiently pass knowledge to them, try to make Tom and Jill help each other as they learn, accent to the team their first good contribution, even if it has cost humongous amount of help from your side. I have seen many onboardings and have been onboarded a couple of times. From what I can tell it's mostly the organization fault when something goes south.
Recruitment will go wrong sometimes, there's no guaranteed way to prevent that. As companies get larger the chance of getting somebody on the team, with whom's interview you weren't involved, gets higher. Sometimes you just have to make the best of these bad situations.
I'm not saying you shouldn't try to fix the hiring problem, just that if something goes wrong try to fix it as much as possible with onboarding, via mentoring.
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