Les cinq cartes Thistledown Duo, Gravelgill Duo, Emberstrike Duo, Tattermunge Duo et Safehold Duo, de l'édition Shadowmoor, forment un cycle appelé "Duo".
Ce cycle a inspiré un cycle similaire, paru dans l'édition Bloomburrow (voir cette anecdote).

Source 1 ( #19) Emberstrike Duo - "[...] +1/+1 bonus seems odd for certain colors. While we wouldn't do the bonus in isolation on certain colors, power / toughness boosting is enough of a basic ability that it's pretty much shared by all colors at a low level, and so we felt fine using it on a cycle. [...]") - Source 2 ( #56) Gravelgill Duo - "[...] The other question this cycle generated was why one color produced a cumulative effect (that is, one that mattered if it triggered multiple times in a turn) and the other side did not. The answer is that there just aren't that many cumulative effects, especially in creature keywords which is what we wanted to grant. [...]" - #72) Safehold Duo - #90) Tattermunge Duo - #93) Thistledown Duo ) - Source 3 (The Glue - "[...] We ended up using an idea we'd talked about in Lorwyn design, what we call duos. The idea of a duo is that it represents two creatures working together. This allows us to put two creature types on the card, making it attractive to drafters drafting either creature type. [...] This also helps with boosting the as-fan of each creature type.")