Yasmin,
The usual advice is never to split a tree because sooner or later,
someone will cross over from one branch to another. I've just found
one instance so far but I'm sure that as soon as I split (not that I'm
going to!) I'd find another crossover.

If it's just a case of wanting easier methods to navigate, there are
several things that you can do. Firstly on the Individuals tab, you
can use the filters to trim stuff down to only show one family (and
those who married into it, providing the options are set right, I
think). E.g. set Filter for last name to XXX.

If that doesn't help - e.g. one family in my tree named every
first-born son George Jackson, then you can configure the columns of
the Individuals tab to show the first spouse (I presume it's not the
default but I set it so long ago, I've forgotten). If you right click
on the grey column headings, you'll see an option "Configure Columns"
or right click in the columns (apart from the first) when you'll see a
"Columns.." option. Right down at the bottom, you'll see that you can
add "<Other...>". That brings up the ability to "Insert Data Ref",
which enables you to add all sorts of stuff - it might be best to talk
about that in the FH User Group site because we can add pictures
there.

Another possibility to help navigation is to follow the "Old picture
paints a thousand words" saying and use the Charts. The one you seem
to want is "Ancestors & Descendants". Select your GF and use him as
the root for an "Ancestors & Descendants" chart (the icons are on the
tool bar). That will show all your GF's siblings and it will start by
showing the siblings' children as well. What you need to do is click
the expansion button below each set of parents to close off the
children for those parents. You can do the same thing with the
children born to your great-grandparents' siblings - close them off by
clicking the expansion button. At some point you can then stop the
tree from going higher by clicking the expansion button immediately
above your GG-GPs (say). Or even close off your G-GF's parents but
keep your G-GM's parents open. Once you've done this (and it's all
reversible by clicking again) then save your work using the menu
option Diagram / Save Diagram As / Family Historian Chart. You should
use Save as a Chart, not the others, because then that can be worked
on again. Open it again by menu option View / Charts / Saved Charts.

The above is about navigating around your stuff - if you want to do
cross checking, then as Mike says, perhaps if you can give us some
instances because there are all sorts of ways to check - just as there
are all sorts of things to check.

Adrian