In Turkish the suffix -(İ)ş is attached to transitive and intransitive verbs and it conveys a meaning of an action being done together. When it combines with a transitive verb like öpmek, to kiss, it shows reciprocity. For example
|John Mary'yi öptü, Mary John'u öptü. Öpüştüler.||John kissed Mary, Mary kissed John. They kissed.|
Some verbs in Turkish have -(İ)ş at the end by default, and they cannot take another -(I)ş. For example barışmak means to make peace, and there is no barışışmak.
Also some other verbs like tutmak (to hold, to keep) can take -(İ)ş but they can have a very different meaning, tutuşmak means to catch fire. Therefore you need to be careful when using this suffix, it might not mean what you think it means.