If two way trust is already set. Then you can run below stsadm commands from your server:
First, you have to set an encryption key on all web servers in your farm so that the stored credentials can be encrypted:
Just to be clear, you have to run this on all web servers in your farm and the "AnyKey" value has to be set to the same value on all of those servers.
STSADM.exe -o setapppassword -password AnyKey
where "AnyKey" is any value.
STSADM.exe -o setapppassword -password B1gL0ngPassw0rd
Next, you have to run below stsadm command to search users from other domain in people picker and give them access
stsadm -o setproperty -pn peoplepicker-searchadforests -pv "forest:DomainA.com;forest:DomainB.com,DomainB\login,password" -url "Your webapp url"
*DomainA: is a domain on which SharePoint installed
Domain B: is a new domain which you have to connect to sharepoint
Once above command runs successfully you will able to search users from DomainB in sharepoint portal which is on DomainA.
Hope this helps!!!!!