Hello Sam,
Topaz and Yellow Sapphire are completely different gemstone as their chemical and physical properties are different. However, they may appear similar and acts as a substitute for one another. Generally, topaz is used instead of yellow sapphire in astrological purpose when one is low on funds. Which is not bad. Even I suggest people to wear Topaz first before investing lots of money in yellow sapphire; so that one can actually test if the gemstone is working fine for their needs or not.
Lets come to your question. Yes, one can wear ruby, emerald, topaz or yellow sapphire together on the same hand (right hand in case of male & left hand in case of female).
I hope this answers your question.