I haven't seen functionality to view multiple channels at the same time in any modern chat application, so I can't exactly blame discord for not having that.
replyWeechat does it: https://weechat.org/files/doc/stable/weechat_quickstart.en.h... I don't use this feature, though.
replyHowever I like that I can see all the channel with unread messages at once, and can switch between them quickly. On Discord if I'm chatting in three channels spread across two servers then switching takes two clicks.