It is indeed playable offline. You would just have to hide your hands from each other since it would be visible to both normally.
Summoner Wars Compendium Stats
Total Games Played: 6,569 (49.21% overall win rate)
Regular Games Played: 2,241 (57.6% win rate)
Custom Games Played: 3,417 (40.6% win rate)
Other: 911 (61.0% win rate)
(Updated May 10)
The night's shadow exists, not sleeping at all,
The toil of the light is its only call,
Behind the scenes lurking with shadowy gall,
Its face never showing; height never too tall.