Sucuri and Ithemes Security together?

I would say it is redundant, but if you are not experiencing any incompatibility between them you can keep them active at the same time, they both have similar features. Keep in mind that the Sucuri WordPress plugin is provided as a complementary tool for our premium services [1][2] you should consider to have at least an antivirus or firewall to have full coverage.

[1] https://sucuri.net/website-security-platform/
[2] https://sucuri.net/website-firewall/