Forging relationships with a competitor gives an IT consultancy the ability to access technical expertise that might otherwise prove unavailable.
The time might arise when you need to "borrow" expertise or manpower from another technology consulting firm; it's called subcontracting, and it happens all the time.
The idea of your consultancy reaching out to competitors for help might sound crazy, but the truth is your shop will never be the experts in everything. Also, the thought of teaming with a competitor shouldn't prove threatening; in my experience, there's plenty of work to go around, and I've seen these relationships work well.
Don't discount a potential partner because of a client's criticism
When you're trying to select another technology consultancy for a partnership, I suggest that you don't base your decision on who not to contact solely on what a client says about the other firm.
Years of working as a consultant have taught me a number of real-world lessons. One lesson is to never assume everything a client tells you about the previous consultancy's staff is true. For example, a client might say the previous consultant never told them to renew an SSL certificate or antivirus license. When the truth comes out, it might turn out the consultant recommended the renewal, but the client refused to pay the corresponding bill, thus it never got done.
Another lesson I've learned is you may believe the previous staff or consultant missed or deployed something incorrectly, when in fact, they didn't make an error. For example, maybe you stumble upon router ports being forwarded to an address that doesn't exist, and later on, you discover that the security system in question was removed from the network.
Share a meal with your competitor
The trick is to reach out to a competitor in a way that doesn't compromise your office. You should offer to take another technology consultant to lunch. If all goes well, you should find yourselves sharing horror stories (without revealing clients' names) and reviewing best practices (without giving away your firm's secret sauces). I like to share a meal with my competitor before reaching for a standard nondisclosure agreement.
Side benefit to partnership: referrals
In many cases, my office has received referrals from competitors -- good referrals, too, not just the dregs they didn't want to touch. In some of the instances, a customer needed a half-dozen desktops, and the competitor just concentrated on servers. In another case, a competitor wasn't comfortable managing a multiple server deployment in which numerous MPLS circuits and a complex VPN were required.
Words of warning
Reach out to a competitor, share a few meals, and you may discover you can forge a friendly partnership. You should, however, tread carefully. Don't reveal secrets or weaknesses or divulge sensitive information. But as the relationship progresses, and as your office increasingly cooperates with another firm, a simple nondisclosure agreement can help set the ground rules and keep everyone safe.