What is "Tachtav" referring to?
Rashi: It is referring (not to the what is underneath the Merkav, but) to the Merkav that is underneath the Zav, to teach us that, as opposed to Mishkav and Moshav, Merkav renders Tamei Adam, but not the clothes that he is wearing.
And what is the latter half of the Pasuk coming to teach us?
Rashi #1: With reference to all the cases in connection with a Zav - his Zivus, his spit, his Zera and his urine, his Mishkav and even his Merkav render Tamei Adam and the clothes he is wearing via Masa.
Rashi #2 (in Shabbos,82b): It is referring to Even Mesame - If a Zav is sitting on large a stone which, in turn, is supported by four posts and touching Keilim that are placed underneath it, the Keilim are Tamei. 1
Even though they are not supporting the weight of the stone on which the Zav is sittting.