You could load the
mtpro2 (Mathtime Pro 2) to generate superwide tildes. An MWE (minimum working example) that shows how to do this is given below. First, though, some comments about this package: In addition to letting you do lots of really cool and useful things, this package also allows you create a summation symbol that's a full two inches (5 centimeters) [!!] tall. The font's creator issues the following warning regarding the use of such a symbol:
... thereby assuring yourself (as well as the designer of the MathTime fonts) the lasting enmity of journal editors everywhere. [Direct quote from p. 14 of the package's user guide]
This warning applies not only to using super-tall summation symbols; it certainly applies to super-wide tilde and hat symbols as well. Put differently: Just because the
mtpro package lets you create certain questionable things (such as super-tall summation symbols and super-wide tildes) doesn't mean that you have to do them.
Here, then, is an MWE that uses the
mtpro2 package to place a ridiculously wide tilde over both
Addendum: The full
mtpro2 package is not free. However, its so-called
lite subset -- which is all that's required to produce the rather dubious effects shown above, is free. This package is not on the CTAN, but it may be downloaded from this site.
Second addendum (March 2013): The
mtpro2 package can also be used to produce super-wide "hat" symbols, by using its