you both need the Standalone, so you have to spent another 25 euro/dollar or whatever. My girlfriend and me play on the same IP and we dont have had any problems yet.
She plays over WIFI with her Laptop and I over Ethernet with my desktop PC.

@deewd22
Were you playing on the SAME server. We've had this issue and are able to all play at the same time on DIFFERENT servers. If we try to play on the same server (i.e. together) only the last person connecting is able to play as the others are automatically kicked.

@DJ yes we can and we do play on the same server, had no issues yet.
I guess it has something to do that she plays with WIFI and iam online over ethernet(cable with RJ45 connection). Maybe she gets a slightly different IP because of the W-lan router which is not included into the normal router. It´s a second WIFI router which is connected to the normal router. Maybe it gives her a different IP which allow us to play on the same server.

wow guys, wow. You CAN play on the same server. I have 3 PC's in my house and we all play together on the same server over a WIFI connection. It has to do with your NAT/PAT settings on youre router OP. Fix that and you should be fine.

Hello,

Please perform the following steps:

Add -port=25007 into your DayZ launch options on one of the PC's. To do this, you can right click on DayZ in Steam --> Properties --> Set Launch Options and enter "-port=25007" (without the quotation marks). The number doesn't really matter as long as it's different from the other computers which will use 2302, 2303, 2305 by default.

Alternatively, if you have access to the router settings and upnp is turned on, you can try turning it off, restarting the router then trying again.

Regards,

JStewart