Unfortunately, it is almost impossible to operate crossband repeat legally. The ARRL's has an article called 'How are you controlling your repeater' that discusses this. It is available on the web as a PDF.
From the ARRL's web site it appears to only be available to members.
I did find a copy here:http://www.kg8id.com/files/rcontrol.pdf
So, if you can find a way to make it legal:
For the purpose of this discussion, let's say that we have two frequency configurations. One we will call A and the other we will call B. A is the .70m frequency that your HT will be accessing / listening to. B is the configuration for the 2m frequency you actually want to access (ex: your favorite repeater).
(You definitely cannot legally control using a 2m frequency and access a .70m frequency)
Set up config A on a .70m frequency suitable for remote control. Be careful in choosing this frequency. You don't want to interfere with other repeaters / stations. You also probably want to set up the 635 to use the tone squelch on this configuration. Then configure your HT to transmit the configured tone. This will in most cases prevent you from crossband repeating a stray signal that just happens to appear on that frequency. You would probably want to set this configuration to low power (5 watts). Why blast a high power signal on this frequency? You will most likely be fairly close to your crossband repeater.
Press the band button so you can set up configuration B. Set up config B for the 2m frequency you wish to gain access to via crossband repeat. So, if you are hitting a repeater have the offset and any tone settings set correctly for the repeater. You should set the power on this configuration to the minimum required to reach your target.
Initially it does not matter whether you start with config A (the frequency your HT will be accessing/listening to) on the left or right side of the display in the 635.
Enable cross band repeat (turn off the radio, hold down the band button, turn on the radio).
Basically, the radio now listens on both frequencies. Whenever squelch is broken on either configuration it is listening on, it retransmits the signal on the other configuration. And yes the configurations do swap between the right and left side of the display such that one side always holds the configuration that is transmitting.
You cannot transmit from your control frequency until the carrier drops from the repeater (or other station on .70m). So you are at the mercy of others. If the carrier never drops, you won't be able to transmit.
Turning off crossband repeat is the same as turning it on. Turn the radio off. Hold the band button. Turn the radio on.
You will probably find it impossible to operate crossband repeat legally. It really is a shame. The 635 really does crossband repeat well.
I hope this helps,