You need to speak with your surgeons, for you seem to have unfounded concerns, as well as misunderstanding of the exact problem(s) and proposed solution(s). Let's do a bit of basic explanation first.
Diastasis recti is stretching of the rectus abdominis muscle sheath (the gristle or fascial layer that surrounds the six-pack muscles in the abdomen). These muscles are the ones that are stretched apart with pregnancy, obesity, and occasionally other rare conditions such as collagen disorders. A hernia is an actual tear, rip, or hole-like opening in this sheath, through which fat or even loops of intestine can protrude, depending on the size of the hernia. Frequently with pregnancy women develop a small umbilical hernia, and their belly button becomes an "outie" instead of its former "innie." When these are small and asymptomatic, they are only cosmetic annoyances and are routinely repaired by any ABPS-certified plastic surgeon during cosmetic abdominoplasty or tummy tuck.
When there is a larger umbilical hernia, a loop of intestine could become entrapped in your hernia, causing strangulation of the bowel, loss of circulation, possible perforation, peritonitis, and severe repercussions, including death. While this is rare, this is why large(r) hernias are repaired, and are covered by insurance. This also applies to splits in the fascia (hernias) other than at the umbilicus. Depending on size, same potential risks and same recommendation: REPAIR.
If your hernia is large enough to require (or recommend) mesh reinforcement, perhaps this is why your plastic surgeon has asked for the participation of a colleague general surgeon.
Many plastic surgeons are fully trained and board-certified general surgeons, as they completed this training before sub-specialty training and board-certification in plastic surgery. While they may never practice the full range and scope of general surgery once they become plastic surgeons (and may not renew their certification in General Surgery), we do not "forget" our ability to do this type of abdominal wall repair. In fact, when general surgeons are unable or fail in their attempts to repair abdominal wall hernias (some of them massive), they call on plastic surgical colleagues to perform more advanced reconstructive techniques than repair alone, or repair and mesh reinforcement. This is part of our plastic surgery training and our own certification.
However, some plastic surgeons come from an ENT, or orthopedic surgery, or urology background, and despite full training and certification in plastic and reconstructive surgery (or simply because they want to let the reconstructive portion of the abdominal wall hernia repair be performed by a general surgeon, and be reimbursed by your insurance) they will ask a general surgical colleague to step in and do this portion of your cosmetic tummy tuck. It may not even be a training or background issue, it may simply be the way referrals are taken care of in your hospital or by your surgeons.
There ARE slightly higher risks of infection or re-operation when mesh hernia repairs are done in conjunction with cosmetic tummy tuck. These are significantly higher if you are a smoker, use any nicotine products (including gum, spray, or patch), or even are exposed to second-hand smoke!
You should not be afraid, but this reassurance should come from both your general surgeon and your plastic surgeon. They owe it to you to explain to your satisfaction just what is being done, by whom, and with what risks. Please call them both tomorrow, or reschedule surgery when all of your questions are answered.