There are few helpful instruction to guide you while
potty/toilet training boys and girl, each in in their own special way:

Girls:


Try using a potty chair which will allow your daughter’s feet to touch the
floor. This will help which relax the pelvic muscles. When you decide to step
up to using an adult toilet, give her a step stool.

To reduce the chances of sprinkling and making a mess,
have your daughter sit all the way back so that the potty opening is completely
covered by her lower body. Encourage her to sit with her knees apart, which
will also help relax her pelvic muscles.

Teach your daughter to wipe from front to back (wiping from back to front may
introduce fecal material into the urinary tract and/or vagina, resulting bacterial
in infections) by letting her imitate you. Make it fun, as much as possible: keep
her entertained and sitting with books, stickers, or music next to the potty.

Boys:

Let your son him use a potty chair to pee especially in
the initial trials as he is not ready to stand and aim. For pooping, use a
potty chair or toilet-seat insert (plus a step stool). Try sitting first (for
peeing), to simplify the process, especially when you’re training both your
daughter and son together, as they will most likely want to imitate each other.
Later on, you may want to give your son a target that you throw into the
toilet, preferably a circle shaped object (like cereal) and have your son try
to aim at it.

Further down the line of training, have your son push his penis straight down
before he sits on the potty chair to avoid scraping it on the splash guard. If
he's standing, be sure to position him, feet slightly apart, directly in front
of the potty. It’s OK to let him observe his dad, especially knowing that the standing element can be
challenging for potty training boys. When your son is ready to learn to pee
standing up, having him follow daddy around or older siblings may actually
motivate him to perform better.

Continual motivation and reminders are essential
for potty training toddlers, as active toddlers may not want to stop what
they're doing in order to use the toilet or stay on the toilet long enough to
relax and go. Tips for motivating boys include buying "big-kid"
underwear that he'll want to keep clean; for both your twins, having special
toys and books for potty training time, and offering plenty of praise and
prizes.

And remember:

-       Do Not use charts showing an obvious comparison
between your twins’ potty performance. One or two stickers are good enough for
a reward, but putting them on a chart can be a constant visual reminder to one
twin that he/she is “behind” the other.

-       Do Not use one twin’s potty success to
encourage the other. The twin who requires longer time with potty training will
be able to figure out the progress of his/her sibling without you pointing it
out. Instead of being motivating, comparisons might intensify feelings of
jealousy or competition.