Regular open communication. This is easier said that done - no piece of software will solve this for you. It comes from a healthy team culture.

Using chat and tracker tools like Slack, Jira, etc are of course helpful, but it's still easy for designers to get out of whack as they split up problems and tasks over the course of a sprint.

Have formal and informal design show & tells - framing the problem and asking questions and for input from each other on how to solve it together.

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The ever-changing loop Make sure everyone is aware of what the other folks are doing; that everyone understands how the parts will be coming together as the sprint progresses.

When a problem arises or a spec has to change (e.g. API doesn't work that way - Feature B will be pushed to a later phase) make sure everyone is in the loop about it, even if it doesn't really seem like it will affect them.

Agile processes can be difficult for designers - we often look at things in a very different way from developers. Designers want to understand the end goal, to have a picture of what the result is going to be and what it means for the user. Anything that alters that result can severely impact on a designer's process, in ways that other members of an agile sprint team might not fully see - constant sharing is crucial, regardless of the tool or medium you use.

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Don't dismiss the big picture to achieve the small one It's easy to fall into a pattern of focusing all discussion entirely on the sprint, but I think it's very important for designers to take some time to talk about things that fall outside of it - the bits that come up during researching and questioning at the start, but that you don't have the time to address within the sprint.

Make sure to allow some time to step back, discuss the holistic aspects of what you're doing; and meticulously file away the things that surface from such discussions. Present a simple summary of these smaller 'housekeeping' design issues at the end of a sprint, so that they get scheduled properly into the next sprint.

Otherwise, designers will struggle to get the time to realign and refocus the little details that often get loosened up and shaken about during the frantic parts of an agile sprint. If you don't allow for these, you'll find yourself sitting awake at night remembering a million tiny things that you have sitting in a notebook or trello board.