Here's what rhymes with simultaneously.

Consider these alternatives

simultaneous / cutaneous various / areas separate / late instead / said effectively / respectively both / growth thus / us these / disease each / need respective / effective automatically / dramatically thereby / by ways / days either / neither communicate / great appear / will them / then aim / same across / loss positions / conditions same / came then / when moving / removing again / then screens / means using / losing individually / habitually able / table locations / relations