You can have as many Gtks as you like on your system.
I've got one "main" version which is usally the version which comes with my distribution. I rarely touch this version.
Then I've got the latest one all complete with most recent Gtk, Glib, Pango, Cairo, Freetype and so on. Sometimes, there's a third version to play with - add patches for example. Mostly, I put them into /opt and simply add the path to /etc/ld.so.conf and adjust PKG_CONFIG_PATH according to what I need.
It's not that hard to compile all this by hand, BUT:
Gtk-based applications require several additional libraries which themselves require additional stuff. Mostly, they expect a complete environment of freetype, glib, atk, librsvg, poppler, pango, cairo and gtk - so don't be surprised if configure fails and asks for more.
A (very) short overview:
freetype = font handling (TTF stuff and other things),
glib = libc-wrapper,
atk = I forgot the details (some accessibility stuff)
librsvg = SVG handling, used by cairo,
poppler = pdf handling, used by cairo,
pango = text handling beyond just "fonts",
cairo = 2D rendering library and at last
gtk = "all the widgets and the buttons"