Hi:
You can concatenate your two field values together.
So, like this:
string field10 = this.MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:field10", this.NamespaceManager).Value;
string field7 = this.MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:field7", this.NamespaceManager).Value;
string Filename = field7 + " - " + field10;
Of course, you could also add another field to your form that uses the concatenated values as a default, then just reference that field in your code.
Now, I would be remiss if I didn't mention to you that you really, really, really, if at all possible, should rename your fields. If your form is not currently in production, you can rename these without too much trouble, and then, in a few months, when you open this template to work on it again, you'll be so happy you did. If you give all your fields and groups meaningful names, future you will thank present you (as will any other designer who ever needs to work on your form).
If this form is already in production, things get more complicated, because you don't want to risk losing data, and writing a custom upgrade transform is probably a bit advanced. But for next time, be sure to name your fields right from the start.