Like WeatherVane I wouldn't add an extra sign-off.

I wouldn't use "Thanking you" as a sign-off at all, actually, but in a less formal communication I might use "Thanks" or "Many thanks" or "Thank you" as a sign-off (with a following comma). (This would then replace rather than supplementing the "Yours sincerely".)

In a formal letter, instead of having a thank-you as a sign-off, I'd put something like "Thank you very much for any assistance you can provide", "Thank you in advance" (for help not yet given), "Thank you very much for all your help", "Once again, I am grateful for your assistance", "Thank you once again", or "Once again, thank you very much" (followed by a full stop, not a comma) as a complete sentence at the end of the letter (but before "Yours sincerely"). I would probably only use the "once again" versions if I had also thanked them at the start of the letter.

Like WeatherVane I wouldn't add an extra sign-off.

I wouldn't use "Thanking you" as a sign-off at all, actually, but in a less formal communication I might use "Thanks" or "Many thanks" or "Thank you" as a sign-off (with a following comma). (This would then replace rather than supplement "Yours sincerely".)

In a formal letter, instead of having a thank-you as a sign-off, I'd put something like "Thank you very much for any assistance you can provide", "Thank you in advance" (for help not yet given), "Thank you very much for all your help", "Once again, I am grateful for your assistance", "Thank you once again", or "Once again, thank you very much" (followed by a full stop, not a comma) as a complete sentence at the end of the letter (but before "Yours sincerely"). I would probably only use the "once again" versions if I had also thanked them at the start of the letter.