hokuyo_node using 2 hokuyos at the same time

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Thanks for the answer, I shall try the first method you suggested. I now only need to figure out how to manipulate tf frames.

So far I wrote a simple launch file that runs two hokuyo_nodes with different names and usb ports connected. Both of them publish to /scan. I can see data from 2 lasers in rviz (which reads /scan via hector_slam but maps are inconsistent) Now how can I transform the frame of one laser into second?

You can't easily, since laserscans use polar coordinates. I'm not sure about your setup - I assume both lasers publishing to different frames that are properly connected to the tf tree.

You can try the method I suggested where you turn the laserscans into pointclouds, which are Cartesian and can be transformed using tf.

Yes, I have 2 frames: laser and laser_1; the parent for both is base_link; So I added to the launch file this: <node pkg="tf" type="static_transform_publisher" name="base_to_laser_broadcaster" args="0 0 0 pi 0 0 base_link laser_1 1"/>
where pi is 3.14.... for 180 degrees of yaw.

When mapping I get a 360 degree view but it's very very inconsistent and the map is very strange, it looks like several maps overlapping, any ideas on that?

You should ask a new question, providing details on your setup (tf an otherwise), pictures of the laserscans overlaid on each other in rviz to show that your odometry is decent, pictures of your maps, and more details on your issue.