Make sure you two are using 'premium' accounts, by this I mean accounts that are bought from the official Minecraft website. If you buy an account that is labeled as 'cracked', you aren't able to join premium servers. Basically, you need to pay for a premium account to join premium servers, one of which is Manacube.
Use a launcher that is trusted by Mojang; Badlion Client, Labymod, 5zig, Lunar Client & the official Minecraft launcher just to name a few that should all work. If you're getting authentication issues, it gives me an instant red flag that you've either got a cracked account or you're using a cracked launcher (Tlauncher for example) to try to access premium servers.
Any more questions then just reply here, I'm more than happy to assist with launcher/account issues ^w^
Regards,
Jashhy