Can I get to Yangshuo from Guilin city by joining the Li River Cruise tour with two luggage bags?

You can take a Li River cruise to Yangshuo, and I didn't hear there is some specific luggage size limit. The boat leaves GL Zhujiang Dock at 9:00 and will arrive in Yangshuo at around 13:00.
You can also take a train from GL to Yangshuo and there are two high speed trains departing from GL Railway Station at 15:30 and 21:00. There is no luggage size limit either. Upon arrival, take the speical bus line to Yangshuo.
It is suggested you book the train ticket in advance. If you want to do online booking, you may need some help from your Chinese friends, as the website doesn't have English version. You may also contact TravelChinaGuide via bookingtravelchinaguide.com, as they provide booking service for train tickets. May help! :)

Actually, taking a bus or cruise from GL to Yangshuo is accepted by many travelers, as Yangshuo Train Station is not very near the town.
There are frequent buses at GL Passenger Terminal to Yangshuo from 7:00 to 21:00. You can buy tickets when you get to the bus station.