2 separate flight tickets

Yes, of course you can. You can use separate tickets and unrelated airlines for both 24 and 144 hour visa free transit in wider Beijing area. In other words, you can use this option in the range from a couple of hours to up to 6 days, absolutely visa free.

You should just print out your flight tickets and also your hotel booking confirmation and have those documents at hand in both Vienna at the time of check-in and upon your landing in Beijing.

During your check-in in Vienna, you will declare that you are flying to Bankok via Beijing using 24 (if staying in Beijing for less than 24 hours) or 144 hours (if staying in Beijing for more than 24 hours) and after a short check, you will be allowed to board the plane. Upon your landing in Beijing, you will approach transit desk where you will apply for a temporary entry permit for free. There you will again present your documents and after a short check, you will be granted a permit and you will be allowed to exit the airport and to enjoy your time in Beijing on your own, absolutely visa free. You should just take care that you do not have any additional landings within mainland China beside Beijing.