二刀流球员定义
MLB teams must designate every player on the active roster either as a pitcher o
r a position player and are limited to carrying 13 pitchers on the active roster
(14 pitchers from Sept. 1 through the end of the regular season).
大联盟球队必须指定名单中每一个球员是投手或野手，名单中最多只能带13名投手。（9/1
以后到赛季结束时增加为14名）
Those designated as position players are unable to pitch unless it is extra inni
ngs or their team is ahead or trailing by more than six runs when they take the
mound. Teams can also designate players as two-way players if they meet certai
n criteria. Two-way players are able to pitch in any situation but don't count t
oward the active roster's pitcher total.
担任野手的球员不能上场投球，除非是延长赛或者登板时已经落后/领先超过6分。球队也能
指名
达成特定标准的球员为二刀流，二刀流可以在任何情况上场投球，但不占投手的名额。
Players qualify for the two-way designation if they have met both of these condi
tions in either the current or previous MLB season:
于当前或者是前一个球季同时达成下述两项标准的球员可以被指名为二刀流
‧ Pitched at least 20 Major League innings
‧ Played at least 20 Major League games as a position player or designated hitt
er, with at least three plate appearances in each game
在大联盟投至少20局
在大联盟以野手或指定打击身份出赛至少20场，且每场最少要打3打席
来源：https://www.mlb.com/glossary/rules/two-way-players
大概是这样
有二刀流在球队调度会比较有弹性，可是要怎么取得资格感觉会是很大的问题XD