How to run an independent callback function in parallel?

Hi

rosinit
obj = StoredData;

image_sub   = rossubscriber('/rgb/image_rect_color/compressed',{@callbackFn3a, obj}); % Independent, callback occurs on new image receive

depth_sub   = rossubscriber('/depth/depth_registered/compressed',{@callbackFn3b, obj});% Dependent, callback occurs similarly on new depthmap receive
                                                                                       % Why dependent? Because inside callback3b after readImage(message)
                                                                                       % callback3b needs image recieved by image_sub.
% Additional info: image always arrives before depthmap arrives (~ 10ms before)

callback3a

function callbackFn3a(~, message, obj)
    obj.im = [obj.im(2:end,:);...                         % deletes old 1st row; outputting in class object
            {message.Header.Seq, readImage(message)}];    % adds new row in bottom  {image_sequence, image}
 % Additional info: readImage(message) takes ~25ms for rgb image
end

callback3b

function callbackFn3b(~, message, obj)
    depthMap = readImage(message);                        % reads the depth map, ~10ms for grayscale image
    [h, idx] = sort(depthMap(:), 'descend');              % an operation ~10ms, to utilize the timegap till the image in callbackFn3a is ready

    while true
        im_idx = [obj.im{:,1}] == message.Header.Seq;    % matching image_sequence with depthmap_sequence
        if sum(im_idx) == 1
            break                                        % break while loop when finds match
        end
    end                                                  % this while loop is only to wait if readImage in previous callback3a took more time
                                                         % if callback3a was not late then while loop survives for only 1 iteration
                                                         
    im = obj.im{im_idx,2};                               % getting the matched image
    ...
    %%%%%%% rest of code
    ...
end

By the way StoredData is nothing but a class object

classdef StoredData < handle
        properties (SetAccess = public, GetAccess = public)
            im = cell(10,2);                              % callbackFn3a feeds data, callbackFn3b uses data
        end
end

I want to run callback3a in parallel !!!!

Problem I faced currently is due to simultaneous processing in callback3b some of the image sequence is skipped by callback3a.

Can a dedicated worker is assigned to image_sub??

Thanks for consideration