The easiest, cheapest & only way is to share a cabin with yourselves or someone else.
I have travelled frequently on the Marco Polo when it was owned by Orient Lines in cabin 101 or 102, which are really single cabins. I have noticed that Transocean market them as twins, but I cannot imagine where they are puitting the other bed. Try asking your travel agent to try and do a deal, i.e. 1 twin plus one of the above mentioned cabins. Marco Polo was an excellent ship under Orient Lines, but cannot really recommend Transocean as I sailed with them May 2007 and wasn't particularly impressed, but hopefully they have kept the same standards on Marco Polo. Happly cruising, keep looking and you never know what deals you can come with. I know cause I usually sail as a single and refuse to pay more than £100 per week extra for a single cabin.